Iron complexes as potential photocatalysts for controlled radical photopolymerizations: A tool for modifications and patterning of surfaces

Abstract

International audienceThis article reports on the presumably first use of iron complexes (FeC) as potential photocatalysts for controlled radical photopolymerization reactions (CRP2). Three compounds were designed and investigated. Good linear evolutions of the molecular weight (Mn) with the conversion were observed. A comparison was provided with a reference iridium (III) complex [Ir(ppy)3 where ppy stands for 2-phenylpyridine]. The on/off photopolymerization experiments highlight the presence of dormant species and a re-initiation on demand upon irradiation. This unique re-initiation property was used for the modification of surfaces (hydrophilic/hydrophobic properties) and surface patterning as well as the synthesis of a block co-polymer (PMMA-b-PBA). A comparative analysis of the behavior of these iron complexes in thermally and photochemically activated polymerization was provided. The chemical mechanisms were studied by steady state photolysis, laser flash photolysis, cyclic voltammetry, luminescence quenching, and electron spin resonance experiments. A catalytic cycle was proposed with two steps: (i) the oxidation of the FeC excited state by an alkyl halide and (ii) the reduction by the oxidized form (FeC°+) by an amine or the macroradicals leading to the regeneration of the catalyst

Photocatalysts in Polymerization Reactions

“…Copper complexes bearing a bis(2‐(diphenylphosphino)phenyl)ether chelate (Figure ) to enhance their excited‐state lifetimes presented unprecedented photocatalytic activities in the photopolymerization of acrylate and epoxide monomers by using visible‐light LEDs . Even with their low excited‐state lifetimes, iron complexes bearing iminopyridyl ligands (Figure ) acted as efficient redox photocatalysts in FRP, in particular when photosensitive functionalities such as pyrene were linked to the ligands (Figure ) . More interestingly, in the presence of a sacrificial electron donor, such as an amine (Scheme ), a living character was observed for the acrylates and methacrylates radical polymerization by using iron‐based PCs (see below) …”

“…Even with their low excited‐state lifetimes, iron complexes bearing iminopyridyl ligands (Figure ) acted as efficient redox photocatalysts in FRP, in particular when photosensitive functionalities such as pyrene were linked to the ligands (Figure ) . More interestingly, in the presence of a sacrificial electron donor, such as an amine (Scheme ), a living character was observed for the acrylates and methacrylates radical polymerization by using iron‐based PCs (see below) …”

“…Recent articles reported the CRP of methacrylates by using catalytic amounts of iron(II) complexes under visible‐light irradiation. For instance, iron complexes bearing bipyridine or iminopyridine ligands (see Figure ) induced a CRP process with methacrylate monomers and allowed the synthesis of PMMA‐ b ‐P(benzyl methacrylate) or PMMA‐ b ‐P(butyl acrylate) block co‐polymers, respectively. Moreover, controlled surface patterning has been made possible by using Fe II –iminopyridine complexes as CRP photocatalysts …”

Reversible Deactivation Radical Polymerization: State-of-the-Art in 2017

“…Parallel to the reduction cycle for copper mediated photo-ATRP described above, polymerization only took place in the presence of irradiation and was completely suppressed in the absence of light. Similar oxidative cycles were also implemented for photo-ATRP of acrylates and methacrylates using transition metal photocatalysts such as fac-Ir(ppy) 3 (91-95) and iron halides (96)(97)(98)(99)(100)(101)(102)(103). For instance, iron mediated photo-ATRP for polymerization of methacrylates was demonstrated without the need for additional ligands, reducing agents, and radical initiators (96).…”

Iron Catalysts in Atom Transfer Radical Polymerization

“…ATRP catalysis involves generation of radicals via activation of halogen chain ends by L/Cu I activator and reversible deactivation of propagating radicals by a halogen atom transfer from L/Cu II -X deactivator (X = Br or Cl). While copper-based complexes have been widely explored and used for polymerization of a wide range of vinyl monomers with high efficiency, ATRP catalysis comprises other transition metal-based catalysts [16,17] such as iron [18][19][20][21], ruthenium [22], osmium [23], and iridium [24] complexes as well as organic-based photoredox catalysts [25][26][27][28].…”